The present invention relates to a machine position sensing device for sensing the position of a movable machine part in tracer control equipment or the like.
Tracer control equipment is theoretically capable of machining even without the recognition of the position of a movable machine part. In recent years, however, tracer control units have been equipped with a simple position control function and machine position sensing function so as to enable tracing return control and stop control without involving the use of a mechanical switch such as a limit switch or to permit positioning of the movable machine part when tracing is switched from one tracing area to another. These functions are performed through utilization of feedback pulses available from a position sensor associated with a motor directly or indirectly. Incidentally, the detected position of the movable machine part is utilized not only for tracer control but also it is displayed for monitoring the tracing operation by an operator.
In the prior art, the position of the movable machine part is sensed by counting feedback pulses from the position sensor through use of a counter, reading out the count value from the counter at predetermined time intervals, applying a sign-inverted version of the thus read-out count value to the counter to cancel its contents, and adding the above read-out count value to machine position data obtained so far.
When backlash is present in the drive system, however, the feedback pulses from the position sensor will be generated even if the movable machine part is not actually driven. Therefore, the machine position data obtained only by adding up the counter contents being read out does not accurately indicate the position of the movable machine part under the influence of the backlash. In addition, the machine position data frequently changes in response to driving within the backlash, posing the problem that the display of the data flickers and hence is very hard to see.